Which of these numbers is prime? ${23,\ 25,\ 39,\ 63,\ 88}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 23 are 1 and 23. The factors of 25 are 1, 5, and 25. The factors of 39 are 1, 3, 13, and 39. The factors of 63 are 1, 3, 7, 9, 21, and 63. The factors of 88 are 1, 2, 4, 8, 11, 22, 44, and 88. Thus, 23 is a prime number.